Saw dog wearing lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'dog wearing lead' is the wordplay.
'dog' becomes 'rover' (Rover is a common name for a pet dog).
'wearing' indicates putting letters inside.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' placed into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Saying of great truth" , "OT saying" , "Short saying expressing a supposed truth" , "encapsulate wisdom" , "Adage, pithy saying" .)